A digitizer board comprises a substrate carrying two groups of orthogonally disposed grid wires, each group comprising a plurality of spaced parallel conductors. A selectively operable control circuit causes equal current pulses to flow in predetermined sequence and direction through the conductors of one group of grid wires, and then through the other group, producing magnetic flux lines that are coupled to a coil in a measuring sensor when the latter is operatively positioned relative to the board. The resultant induced voltages in the coil are processed to provide the coordinate of the coil relative to the board.
One important application of digitizer boards of the type described is to obtain the coordinates of individual points defining a two dimensional profile on a sheet lying on the digitizer board. In such case, the coil is used as a cursor to permit an operator to visually select individual points on the profile whose coordinates are to be determined.
The spatial location of the point on the digitizer board whose coordinates are determined in the manner described above is coincident with the electrical center point of the coil. That is to say, the coordinates of a point determinded with a toroid are the geometric center of the coil. Thus, when the coil is provided with cross-hairs aligned with the electrical center point of the coil to assist in visually positioning the coil, the degree to which the cross-hairs coincide with the electrical center point of the coil determines the coordinates produced by the processing of the voltages induced in the coil.
Conventionally, a glass plate carrying cross-hairs is adjustably mounted on the coil. After assembly of the glass plate and coil, the plate is adjusted with respect to the coil in an attempt to place the crossing point of the cross-hairs at the electrical center of the coil. While this may be achieved, the time and cost of this manufacturing operation is considerable. It is therefore an object of the present invention to provide a new and improved cursor which does not rely on any adjustment to accurately align the geometric center of the coil with its electrical center point, and to provide a new and improved method for manufacturing the cursor.